1. Field of the Invention
The present invention relates to surge protection devices employing thermally responsive devices.
2. Description of Related Art
Thermally responsive devices are often employed to electrically disconnect surge protective components that are experiencing thermal runaway. They are generally placed in close proximity to the component, and trip permanently when a sufficient amount of heat is transferred to the thermal sensing mechanism to bring it to a predetermined temperature threshold. Commonly used thermally responsive devices include thermal cut-offs (TCOs) and bimetallic trip actuators and, less commonly, shape memory alloy (SMA) trip actuators.
TCOs generally take one of two different forms: eutectic and chemical pellet. A eutectic TCO depends on a conductive fusible alloy link with a very abrupt phase change characteristic as it reaches its melting point. A chemical pellet TCO uses a material with eutectic properties to actuate a spring-loaded contact mechanism, and generally has higher operating current ratings than eutectic TCOs (15-25 versus 4-7 Amperes at 120 Vac).
Commonly used surge protective components are placed across electrical poles to limit the interterminal voltage, and include metal oxide varistors (MOVs), silicon avalanche diodes (SADs), gas discharge tubes (GDTs), and thyristor surge protectors. A varistor (MOV), for example, is used to dissipate high-energy power surges by converting excess electrical energy into heat.
MOVs and TCOs are currently the most popular combination in the surge protective device (SPD) industry. The ZnO in an MOV has a melting point significantly higher than Cu (1975° C. vs. 1085° C.). Cu is the primary casing material for chemical pellet TCOs. If a surge has enough energy, it can cause a varistor (MOV) to heat up enough to eject conductive plasma that can cause damage to the surrounding area and additional shorting. That is, An MOV, in response to sustained voltages well in excess of its rated value, can eject hot material (plasma) at high velocity, due to extreme hot spots caused by localized, self-promoting breakdown of granular diode junctions. A direct strike of ZnO plasma to a chemical pellet TCO's casing can cause it to lose its ability to actuate. Likewise, the brass material in a bimetallic circuit breaker conductor and Nitinol SE508 shape memory alloy are also vulnerable in this application, with respective melting points of 900-940° C. and 1310° C.
The primary means of heat transfer in the prior art is through conduction and radiation. Conduction transports heat through conductors in the wiring assembly, and infrared radiation and/or conduction transports heat in the air space or potting compound existing between the components. In a conducted/radiated heat transfer scenario, relative proximity and orientation of the MOV hot spot to the TCO are major factors, and are difficult to control.
In addition, the proximity requirement for conduction/radiation commonly forces a compromise between disconnector safe operation and response time. The chemical pellet TCO needs an insulating barrier to protect it from MOV plasma, which compromises conducted/radiated heat transfer, thus slowing its response to the thermal runaway condition.
ANSI/UL 1449-2006 (Surge Protective Devices) requires abnormal overvoltage testing of these devices for various levels of available supply current to simulate the probable range of thermal runaway scenarios. Most protectors fall into the UL 1449 “Type 3”, or “point of utilization” category, where they are utilized downstream of 120 Vac/15 A branch circuits. UL assigns the following test currents to this category:
1. Limited Current Test. Four levels: 5, 2.5, 0.5 and 0.125 Amperes. These tests are relatively benign when using thermal disconnector coupling as seen in the prior art.
2. Intermediate Current Test. Three levels: 50, 150 and 1000 Amperes. The first two represent especially difficult conditions for thermal disconnectors utilizing conducted/radiated heat transfer. Using the aforementioned method, the disconnect operation is not fast enough to prevent MOV emissions from creating arc paths that bypass the TCO. Arcing in this scenario is self-promoting; as the MOV continues to conduct through these alternate arc paths, it liberates combusted materials from adjacent insulating materials and PCB laminate into the surrounding environment. The two most likely failure modes in this case are excessive ground leakage, due to contaminate deposited on the wiring assembly, and fire.
A supplementary fuse is often used in series with the MOV to speed the disconnecting operation for the intermediate current conditions. That is, a thermal fuse may be used in conjunction with the varistor (MOV) to break the electrical current to the MOV once the temperature of the MOV gets to a certain temperature. However, there are challenges in ensuring that the thermal fuse will open at the appropriate time and will not be shorted out by plasma ejected by the MOV.
A standard, fuse-assisted pellet TCO 1-port SPD is illustrated in FIG. 1, and a standard fuse-assisted pellet TCO 2-port SPD is illustrated in FIG. 2. As can be seen, a fuse F1 is placed in series with the MOV. Current-limiting fuses can carry the load and surge current, but are very expensive if well matched with a modern MOV. A typical part used in this scenario could be a 7-Ampere time delay fuse, which is generally rated too low to be placed in series with the load, and compromises surge capacity. This is a disadvantage for two-port SPDs.
The eutectic TCO has a safety advantage in that the alloy link effectively combines overcurrent fusing with an electrical connection that is inherently un-weldable, unlike the pellet version's mechanical contact configuration. This makes it very effective in clearing the intermediate fault currents. It cannot be welded by a plasma strike, overvoltage fault or surge current. However, the pellet versions generally can handle more load and surge current without opening, and are more often placed in series with the equipment load than the eutectic versions. In this regard, a standard eutectic TCO 1-port SPD is illustrated in FIG. 3, and a standard eutectic TCO 2-port SPD is illustrated in FIG. 4.
Both of these scenarios yield the same result: in the event of an intermediate-level overvoltage fault or large surge current, the protection will be disconnected, leaving the equipment load unprotected. The pellet TCO can disconnect the load for more minor faults, but is compromised by the need for auxiliary fusing and insulating barriers. The eutectic version can only disconnect the protection.
Solutions currently available in the industry tend to leave one or more of these issues unaddressed:
1. The most common method is to place the MOV in close proximity to the TCO, and wrap insulating tape around the component assembly. This serves to keep the parts close together and to prevent some of the heat from escaping quickly into the environment. This is very effective for limited-current situations, but offers little control over the materials emitted from the MOVs when shunting the intermediate currents. The MOV plasma will breach the tape in short order.
2. An improvement over the first method is to replace the insulating tape with a fire-resistant pouch, which more effectively contains the heat and MOV emissions. However, the MOV materials emitted during intermediate current conditions often compromise most flexible insulators, which can provide additional fuel to the fire.
3. Thermally-protected MOVs integrate a low-temperature solder link between one of the leads and the MOV disk. Littelfuse, Inc. offers a version called the “TMOV”, for example. They are reasonably effective at coupling the heat to the thermally responsive device, but cannot carry typical equipment load currents, and require assistance from an external fuse for the intermediate current tests. They are also more expensive than MOV/thermal cut-off combinations where multiple MOVs are thermally coupled to a single discrete TCO, which is a more typical situation in the field. US 2007/0200657 is directed to a thermally protected MOV with a built-in overcurrent fuse that may address the intermediate current issue, but does not appear to be rated to carry or interrupt typical equipment load currents.
4. Ceramic box-encased MOV assemblies place one or more MOVs inside a fire-resistant, sealed enclosure, with air or potting compound (such as silica or a thermosetting resin) used as fill. The MOVs may have a ceramic coating. This configuration is effective at containing the arc-promoting residues emitted by the MOVs during thermal runaway. A version of this is the “X3” produced by Energetic Technology Co., with three ceramic-coated MOVs placed in free air inside a ceramic enclosure. The TCOs are located outside the box. Because ceramic materials are such good thermal insulators, and because the MOV hot spot locations are extremely difficult to predict, the entire box would have to come up to a temperature sufficient to radiate enough heat to trip the external TCO(s), which takes additional time. US 2007/0290786 is directed to such a device, including an improvement for thermal coupling using an additional enclosure and TCO mounting features, but only partially mitigates the thermal time response issue.
5. Ferraz Shawmut Inc. produces another enclosed MOV assembly called the “TPMOV”. It integrates a spring-loaded “arc shield”, which is held in place by one of the MOV electrodes. A conductive fusible alloy alloy bonds the electrode to a flat lead; when the alloy melts due to thermal stress, the “arc shield” slides between the lead and the electrode, and effectively quenches the arc. This configuration has excellent response time and fault current capacity. This part was designed for primary power applications (located at the service entrance panel). However, they do not have a version priced suitably for the majority of point-of-utilization SPDs, and the present configuration does not directly disconnect the load.